The present invention relates generally to implantable pulse generators, e.g., a pulse generator used within a Spinal Cord Stimulation (SCS) system or other type of neural stimulation system. More particularly, the present invention relates to the use of a rechargeable zero-volt technology lithium-ion battery within such an implantable pulse generator.
Implantable pulse generators (IPG) are devices that generate electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a spinal cord stimulation system. A spinal cord stimulation system is a programmable implantable pulse generating system used to treat chronic pain by providing electrical stimulation pulses from an electrode array placed epidurally near a patient's spine. SCS systems consist of several components, including implantable and external components, surgical tools, and software. The present invention provides an overview an SCS system and emphasizes the use of a rechargeable zero volt technology battery within such a system, including the charging system used for charging the rechargeable battery.
Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. SCS systems typically include an implantable pulse generator, lead wires, and electrodes connected to the lead wires. The pulse generator delivers electrical pulses to the dorsal column fibers within the spinal cord through the electrodes implanted along the dura of the spinal cord. The attached lead wires exit the spinal cord and are tunneled around the torso of the patient to a subcutaneous pocket where the pulse generator is implanted.
Spinal cord and other stimulation systems are known in the art, however, to applicants' knowledge, none teach the use of a rechargeable zero-volt technology battery within the implanted portion of the system, with accompanying charging and protection circuitry, as proposed herein. For example, in U.S. Pat. No. 3,646,940, there is disclosed an implantable electronic stimulator that provides timed sequenced electrical impulses to a plurality of electrodes so that only one electrode has a voltage applied to it at any given time. Thus, the electrical stimuli provided by the apparatus taught in the '940 patent comprise sequential, or non-overlapping, stimuli.
In U.S. Pat. No. 3,724,467, an electrode implant is disclosed for the neural stimulation of the spinal cord. A relatively thin and flexible strip of physiologically inert plastic is provided with a plurality of electrodes formed thereon. The electrodes are connected by leads to a RF receiver, which is also implanted and controlled by an external controller. The implanted RF receiver has no power storage means, and must be coupled to the external controller in order for neural stimulation to occur.
In U.S. Pat. No. 3,822,708, another type of electrical spinal cord stimulating device is shown. The device has five aligned electrodes that are positioned longitudinally on the spinal cord and transversely to the nerves entering the spinal cord. Current pulses applied to the electrodes are said to block sensed intractable pain, while allowing passage of other sensations. The stimulation pulses applied to the electrodes are approximately 250 microseconds in width with a repetition rate of from 5 to 200 pulses per second. A patient-operable switch allows the patient to change which electrodes are activated, i.e., which electrodes receive the current stimulus, so that the area between the activated electrodes on the spinal cord can be adjusted, as required, to better block the pain. Other representative patents that show spinal cord stimulation systems or electrodes include U.S. Pat. Nos. 4,338,945; 4,379,462; 5,121,754; 5,417,719 and 5,501,703.
The dominant SCS products that are presently commercially available attempt to respond to three basic requirements for such systems: (1) providing multiple stimulation electrodes to address variable stimulation parameter requirements and multiple sites of electrical stimulation signal delivery; (2) allowing modest to high stimulation currents for those patients who need it; and (3) incorporating an internal power source with sufficient energy storage capacity to provide several years of reliable service to the patient. Unfortunately, not all of these features are available in any one device. For example, one known device has a limited battery life at only modest current outputs, and has only a single voltage source, and hence only a single stimulation channel (programmable voltage regulated output source), which provides a single fixed pattern to up to four electrode contacts. Another known device offers higher currents that can be delivered to the patient, but does not have a battery, and thus requires the patient to wear an external power source and controller. Even then, such device still has only one voltage source, and hence only a single stimulation channel, for delivery of the current stimulus to multiple electrodes through a multiplexer. Yet a third known device provides multiple channels of modest current capability, but does not have an internal power source, and thus also forces the patient to wear an external power source and controller. It is thus seen that each of the systems, or components, disclosed or described above suffers from one or more shortcomings, e.g., no internal power storage capability, a short operating life, none or limited programming features, large physical size, the need to always wear an external power source and controller, the need to use difficult or unwieldy surgical techniques and/or tools, unreliable connections, and the like. What is clearly needed, therefore, is a spinal cord stimulation system that is superior to existing systems by providing longer life through the use of a rechargeable battery, easier programming and more stimulating features in a smaller package without compromising reliability.
Regardless of the application, all implantable pulse generators are active devices requiring energy for operation, either powered by an implanted battery or an external power source. It is desirable for the implantable pulse generator to operate for extended periods of time with little intervention by the patient or caregiver. However, devices powered by primary (non-rechargeable) batteries have a finite lifetime before the device must be surgically removed and replaced. Frequent surgical replacement is not an acceptable alternative for many patients. If a battery is used as the energy source, it must have a large enough storage capacity to operate the device for a reasonable length of time. For low-power devices (less than 100 μW) such as cardiac pacemakers, a primary battery may operate for a reasonable length of time, often up to ten years. However, in many neural stimulation applications such as SCS, the power requirements are considerably greater due to higher stimulation rates, pulse widths, or stimulation thresholds. Powering these devices with conventional primary batteries would require considerably larger capacity batteries to operate them for a reasonable length of time, resulting in devices so large that they may be difficult to implant or, at the very least, reduce patient comfort. Therefore, in order to maintain a device size that is conducive to implantation, improved primary batteries with significantly higher energy densities are needed. However, given the state of the art in battery technology, the required energy density is not achievable at the present time.
If an implanted battery is not used as the power source, then a method is required to transcutaneously supply power to the IPG on a continuous basis. For applications that require large amounts of power such as heart pumps and other heart-assist devices, an external power source is the preferred choice. Power can be supplied to the device via a percutaneous cable, or more preferably and less invasively, coupled to the device through electromagnetic induction. The external power source can be an AC outlet or a DC battery pack, which may be recharged or replaced with new batteries when depleted. However, these systems obviously require the patient to continually wear an external device to power the implanted pulse generator, which may be unacceptable for many patients because they are often bulky and uncomfortable to wear, and naturally, limit patient mobility.
One alternative power source is the secondary, or rechargeable battery, where the energy in these batteries can be replenished by recharging the batteries on a periodic basis. It is known in the art to use a rechargeable battery within an implant device. See, e.g., U.S. Pat. No. 4,082,097, entitled “Multimode Recharging System for Living Tissue Stimulators”, and applicant Carla Mann Wood's U.S. patent application Ser. No. 09/048,826, filed Mar. 25, 1998, entitled “System of Implantable Devices For Monitoring and/or Affecting Body Parameters”, now U.S. Pat. No. 6,208,894 which patent and patent application are likewise incorporated herein by reference. The devices and methods taught in this patent and application, however, comprise specialized devices, e.g., microstimulators, or relate to specific applications, e.g., cardiac pacing, which impose unique requirements not applicable to many IPG applications. Cardiac pacemakers with rechargeable batteries have been developed in the past; see U.S. Pat. Nos. 3,454,012; 3,824,129; 3,867,950; 3,888,260; and 4,014,346. However, these devices were met with limited success in regards to battery performance and market acceptance. Many of these devices were powered by nickel-cadmium (NiCd) batteries. NiCd's low volumetric energy density of 100 Wh/liter provided limited energy storage, and frequent charging was required. Also, its low nominal cell voltage of 1.2 V required many cells to be stacked in series, requiring cells to be closely matched for optimum performance. NiCd batteries also suffered from a phenomenon called “memory effect,” which causes the cell to lose capacity if cycled at shallow discharge depths. Moreover, NiCd batteries have a high self-discharge rate, losing approximately 30% of their capacity per month at body temperatures. Also, cycle life performance was poor, as NiCd batteries typically lasted fewer than 300 cycles. In addition, charging NiCd batteries was often problematic because the standard charge termination method for NiCd batteries is somewhat complicated, requiring the need to detect a zero or negative voltage slope (dV/dt) and/or temperature slope (dT/dt). When NiCd batteries are overcharged, an exothermic reaction occurs: oxygen gas given off at the nickel electrode recombines with the cadmium electrode to form cadmium hydroxide. Cell leakage or venting can occur as a result of the pressure increase in the cell. Furthermore, there may be disposal issues with NiCd batteries, as cadmium is highly toxic to the environment.
Newer battery technologies have been developed in recent years. The Nickel Metal-Hydride (NiMH) battery was developed to improve upon NiCd performance. NiMH batteries were first commercially introduced in 1990, and are in many ways similar to NiCd batteries. The main exception is the replacement of the cadmium electrode with a metal-hydride alloy, resulting in more than twice the volumetric energy density (>200 Wh/liter). In addition, the metal-hydride is less toxic than cadmium. However, NiMH batteries suffer from some of the same drawbacks as well, including low cell voltage (1.2 V), high self-discharge (>30% per month), difficult charge termination, low cycle life (<300 cycles), and to a lesser extent, memory effect.
Rechargeable lithium-based batteries were first developed in the 1970s using lithium metal as the active electrode material. Lithium has great promise as a battery material because it is the lightest of all metals, with high cell voltage (>3 V) and high energy density. However, lithium metal in its pure form is extremely reactive, and proved to be very unstable as a battery electrode as employed in early designs. In 1990, however, Sony Corporation introduced a safer rechargeable lithium-based battery called lithium-ion (Li-ion), which used a lithium composite oxide (LiCoO2) cathode and a lithium-intercalating graphite anode. Lithium ions, or Li+, instead of lithium metal, are shuttled back and forth between the electrodes (hence the nick-name, “rocking-chair” battery). Lithium-ion is superior to other rechargeable battery chemistries, with the highest volumetric energy density (>300 Wh/liter) and gravimetric energy density (>100 Wh/kg). In addition, Lithium-ion batteries have a high nominal voltage of 3.6 V, as well as low self-discharge (less than 10% per month), long cycle life (>500), and no memory effect. Charge termination for Lithium-ion batteries is also simpler than that of NiCd and NiMH batteries, requiring only a constant voltage cutoff. However, Lithium-ion batteries are not as tolerant to overcharging and overdischarging. If significantly overcharged, Lithium-ion batteries may go into “thermal runaway,” a state in which the voltage is sufficiently high to cause the electrode/electrolyte interface to breakdown and evolve gas, leading to self-sustaining exothermic reactions. As a result, cell leakage or venting can occur. If Lithium-ion batteries are over-discharged (<1 V), the negative electrode may dissolve and cause plating of the electrodes. This can lead to internal shorts within the cell, as well as possible thermal runaway. Therefore, careful monitoring of the cell voltage is paramount, and battery protection circuitry is necessary to keep the cell in a safe operating region.
It is known in the art to use a Lithium-ion battery in an implantable medical device, see. e.g., U.S. Pat. Nos. 5,411,537 and 5,690,693. However, such disclosed use requires careful avoidance of overcharge and overdischarging conditions, as outlined above, else the implant battery, and hence the implant device, is rendered useless.
The most recent development in rechargeable battery technology is the Lithium-ion polymer battery. Lithium-ion polymer batteries promise higher energy density, lower self-discharge and longer cycle life compared to conventional aqueous electrolyte Lithium-ion batteries. Its chemical composition is nearly identical to that of conventional Lithium-ion batteries with the exception of a polymerized electrolyte in place of the aqueous electrolyte. The polymer electrolyte enables the battery to be made lighter and thinner than conventional Lithium-ion batteries by utilizing foil packaging instead of a metal can, thus allowing it to be conformable to many form factors. Lithium-ion polymer batteries are also theoretically safer since the polymer electrolyte behaves more benignly when overcharged, generating less heat and lower internal cell pressure. Unfortunately, commercial realization of Lithium-ion polymer batteries has been slow and fraught with early production problems. Only recently have the major battery manufacturers (Sony, Panasonic, Sanyo) announced plans for Lithiumion polymer battery production.
What is clearly needed for neural stimulation applications is a physically-small power source that either provides a large energy reservoir so that the device may operate over a sufficiently length of time, or a replenishable power source that still provides sufficient energy storage capacity to allow operation of the device over relatively long period of time, and which then provides a convenient, easy and safe way to refill the energy reservoir, i.e., recharge the battery, so that the device may again operate over a relatively long period of time before another refilling of the reservoir (recharging of the battery) is required.